1. Field of the Invention
The present invention relates to a nozzle for cold spray and a cold spray apparatus using the same. More specifically, the invention relates to such a nozzle for cold spray and a cold spray apparatus using the same, which can minimize clogging phenomenon of a nozzle generated because the inside of the nozzle is coated with powder of soft material when coating with the powder, and prevent abrasion of the nozzle caused by collision of powder of very hard material against the nozzle wall is prevented when coating with powder, thereby making it easy to apply to mass production since the nozzle can be used for a long time, reducing manufacturing costs in mass production by enabling high quality coating for a long period of time, enabling low cost construction of facilities since the powder supplying device needs not a high pressurizing device, and facilitating modifications of processes by adjusting the location of the spray tube to control the speed of powder without controlling the flow rate of gas supply.
2. Background of the Related Art
A cold spray coating refers to a method of coating the surface of an object to be coated by spraying powder at normal or relatively low unheated temperature using supersonic carrier gas, in which small particles (1-50 μm) accelerated by supersonic jet air currents (300-1,200 m/s) are collided and coated on metallic or ceramic boards, and the temperature and the speed of the accelerated gas and the size of the particles are applied as variables of the coating process.
Specifically, such a cold spray coating method is based on the principle that highly accelerated particles collide into unheated boards for coating, so that the coating efficiency differs according to the materials to be coated. The coating efficiency also increases as the speed of the accelerated particles increases. That is, the coating efficiency shows a characteristic of abrupt increase above a certain speed.
Basic requirements for coating by a cold spray coating method using supersonic speed are as follows: A) The temperature of jet air currents must always be lower than the melting point or the softening point of the accelerated particles. B) The size of the accelerated particles must be within a range between 1 and 50 μm. C) The speed of the particles must be within a range between 300 and 1,200 m/s according to the material and the size of particles. In reality, particles are coated with the help of supersonic jet air currents of Mach 2-4 and 1-3 MPa, and, for the type of gas, a gas such as air, nitrogen and helium or a gas mixture that comprises of air, nitrogen, and helium is used. Whatever gas may be used, coating is possible only when the speed of accelerated particles exceed the critical speed (V<Vcrit).
For this reason, the temperature of gas is raised to increase the speed of gas to so as to increase the amount of gas, and a typical De Laval type nozzle as a publicized technology is used to provide supersonic carrier gas. The technology is disclosed in U.S. Pat. No. 6,139,913 which has the configuration depicted in FIG. 8. However, as shown in FIG. 8, before the throat area, the De Laval type nozzle (a convergence-divergence nozzle) mixes the carrier gas provided from the lower part with a gas/powder mixture which is a mixture of gas and powder provided from the left side before a throat area, and then accelerates the resultant mixture.
Accordingly, as shown in FIGS. 9 and 10, the gas/powder mixture provided like this is accelerated gradually through the convergence section of the convergence-divergence nozzle, reaching the speed of sound at the throat area. In this case, the latter part of the nozzle is configured as a divergence type to maintain evenly the mass of the gas passing a specific point after the gas/powder arrive the speed of sound. Like this, the speed of gas which passed the throat area increases to become supersonic speed in the end. The gas flowing at supersonic speed has such a characteristic that the speed expanding outward is faster than the speed accelerated in the backward, since the gas transfers energy in the direction of circumference when compressed toward the axial direction. Using this principle, a convergence-divergence nozzle makes a thrust which is needed to project the gas/powder mixture in the nozzle at a supersonic speed.
However, in case of the method depicted in FIG. 8, since the gas/powder mixture flows in before the throat area, the powder undergoes a process of passing the throat area and being sprayed. In the case where the sprayed powder is comparatively soft like aluminum, the throat area is coated with powder, thus making the throat area clogged in a short time, so that the coating process cannot be performed any more. Consequently, the above method is hard to apply to mass production. In the case where the sprayed powder is very hard like nickel or super-alloy, the speed at the throat area is not more than speed of sound, and so coating is not accomplished, but the throat area is severely abraded due to the collision of the powder, thereby damaging the nozzle, and the modification of configuration of the throat area changes the flow speed, thus consequently altering the processing conditions.
In addition, in case of coating using the apparatus depicted in FIG. 8, the pressure applied to the spray tube which injects the gas/powder mixture provided from the left side of the nozzle must be higher than the pressure of the gas which is provided to the convergence part as carrier gas that is provided from the lower part of the nozzle, and so an additional pressurizing device has to be provided.
Furthermore, as shown in FIGS. 9 and 10, in case of using a publicized convergence-divergence nozzle, even though the location of the spray tube which provides the gas/powder mixture is changed, it can be observed that the final speed of the outlet flow at the outlet end point of the nozzle is not changed. Accordingly, in order to change the speed of the flow to modify process conditions, the amount of the flow of the entire system needs to be changed.